US involvement in Syria since 2011

Here is a recap of Washington's involvement in the Syrian conflict, following the decision by President Donald Trump to withdraw US troops stationed in the country.

- Pressure on Assad -

On April 29, 2011 in response to a bloody regime crackdown against protests, Washington slaps sanctions and assets freezes on several Syrian officials.

On May 19, a day after ordering the first US sanctions on President Bashar al-Assad himself, US president Barack Obama calls on him to lead a political transition or step aside.

Early July, the US ambassador in Damascus, Robert Ford, challenges Assad by visiting Hama, a central city besieged by the army and scene of a massive demonstration against the regime.

On August 18, Obama and Western allies for the first time call explicitly on Assad to stand down.

In October the US announces that Ford has left Syria for security reasons. Damascus recalls its ambassador from Washington.

- Obama ignores 'red line' -

In summer 2013 the Syrian regime is accused of carrying out a chemical attack near Damascus that kills more than 1,400 people according to Washington.

At the last minute Obama decides against bombarding the regime's infrastructure and agrees on a deal with Moscow on September 14 to dismantle Syria's chemical weapons arsenal.

Previously Obama had vowed to act if Syria crossed the "red line" of chemical weapons use.

- Strikes against IS -

On September 23, 2014 the US and Arab allies launch air strikes in Syria against the Islamic State (IS) group, expanding a US-led campaign against the jihadists in neighbouring Iraq.

The biggest contributor to the coalition, Washington eventually deploys 2,000 soldiers in Syria, mostly from special forces, and uses air and artillery power to pound its targets.

In October 2015 the Syrian Democratic Forces (SDF), a Kurdish-Syrian Arab alliance of some 50,000 fighters -- is created.

Dominated by the Kurdish People's Protection Units (YPG) militia, the alliance receives hands-on training and aid from the US, in the form of arms but also air support and intelligence for their operations.

The SDF has since overrun IS in northeastern Syria, including Raqa and a large part of Deir Ezzor province. They continue to fight jihadists in small pockets of eastern Syria.

- Trump takes action -

On February 8, 2018, the coalition says it killed at least 100 pro-regime fighters to fend off an attack on its allies in eastern Syria.

On April 14 with the support of France and Britain, the US launches retaliatory strikes on the regime after an alleged chemical attack a week earlier on the ex-rebel town of Douma in which some 40 people are killed, according to rescue forces on the ground.

Already in April 2017 Trump had ordered air strikes on Shayrat airfield in central Syria, the suspected launch site of an earlier suspected chemical attack which killed as many as 88 people in Idlib province.

- Turkey-US 'coordination' -

On December 14 Trump and his Turkish counterpart Recep Tayyip Erdogan agree to "more effective coordination" between their countries' operations in Syria.

The telephone conversation between the two leaders follows Erdogan's warning that Turkey was planning to launch a new operation within the "next few days" against the YPG militia in northern Syria.

On December 17 Erdogan says he is determined to "remove" the Kurdish fighters if the US does not force them to withdraw.

- Trump pulls out troops -

On December 19 Trump orders the withdrawal of US troops from Syria, saying in a tweet that America had "defeated IS".

The next day as he defends his controversial decision, Trump says the US does not want to be the "Policeman" of the Middle East.

France, Britain and Germany are quick to condemn the decision while Russia says the US president is "right" to withdraw troops from Syria.